The present invention relates to tantalum pellets used in the manufacture of tantalum and niobium capacitors. In particular, the invention is an improved method of doping tantalum and niobium pellets with nitrogen.
This invention relates to an improved method of making tantalum and niobium pellets, and more particularly to the production of such pellets for use in electrolytic capacitors.
The usual method for making tantalum pellets for use in tantalum capacitors includes steps wherein tantalum powder is first pressed or compacted into a pellet. The resulting pressed pellets then undergo a sintering process wherein the pellets are heated in a vacuum. The heating allows the tantalum particles to stick together so they can hold a lead wire.
Following the sintering process, the tantalum pellet is dipped in an acid solution to form a dielectric film on the outer surface of the pellet and the particles within the pellet which is typically tantalum pentoxide. The pellet and the particles within the pellet are then subsequently coated with various other metal-containing materials which form the cathode.
For electrolytic capacitors, the oxygen concentration in the tantalum is critical. When the bulk oxygen content of porous tantalum pellets is above the oxygen solubility limit in tantalum (about 2 at. % at T less than 873 K), oxide phase precipitates appear on the surface of the particles within the tantalum pellets. These precipitates act as efficient crystallization nuclei and concentrators of the electric field in the amorphous tantalum pentoxide film formed by the subsequent anodization. As the specific volume of the crystal and amorphous phases are different, the crystallization causes disruption of the amorphous tantalum pentoxide film. This renders the dielectric film less reliable and degradation of the capacitor occurs. Capacitors made from such pellets may have unsatisfactory life characteristics. Unfortunately, tantalum powder has a great affinity for oxygen and, thus, the processing steps which involve heating and subsequent exposure to air inevitably results in an increased concentration of oxygen. Since the amount of oxygen absorbed will be proportional to the surface area exposed, fine powders with very high CV properties are even more susceptible to the reaction with atmospheric oxygen.
Methods for reducing the oxygen content of tantalum pellets have included nitrogen doping of sintered tantalum pellets. Nitrogen doping prevents the diffusion of oxygen from the ambient atmosphere and from the tantalum pentoxide film to the tantalum pellet, resulting in stabilization of the chemical composition of the dielectric film. As a consequence, tantalum capacitors made from such pellets have excellent thermostability of DCL and capacitance.
In prior art methods, the sintered pellets after doping with nitrogen consist of a mixture of nitride phase TaN and a solid solution of nitrogen in the tantalum phase. In these methods the nitride phase precipitates on the surface of the pellets to similarly interfere with the tantalum pentoxide dielectric properties as with the oxide phase precipitates. This is due to the surface of the tantalum pellet being oversaturated with nitrogen. It would therefore be beneficial to develop a method of nitrogen doping tantalum pellets to reduce their oxygen content following sintering while also preventing the formation of nitride precipitate.
Niobium is a xe2x80x9cdaughterxe2x80x9d element to tantalum. Niobium and tantalum are often found together and share many of the same chemical and physical characteristics and are difficult to separate. Interest in niobium as a capacitor material is largely based upon its relative abundance compared to tantalum and its lower cost.
The major impediment to the use of niobium in capacitors is the very high rate of oxygen dissolving in niobium metal. This causes the capacitor dielectric niobium oxide to degrade rather quickly at moderately elevated temperatures as oxygen is absorbed into the metal. It would therefore also be beneficial to develop a method of nitrogen doping niobium pellets to reduce their oxygen content following sintering while also preventing the formation of nitride precipitate.
It is therefore a primary object of the present invention to provide a method of reducing the oxygen content of sintered tantalum and niobium pellets.
It is a further object of the present invention to provide a method of doping tantalum and niobium pellets with nitrogen which prevents the formation of a nitride precipitate on the surface of the tantalum or niobium pellet.
It is yet a further object of the present invention to provide an improved nitrogen-doped tantalum or niobium pellet with improved DCL characteristics and reliability.
It is still a further object of the present invention to provide an improved nitrogen-doped tantalum or niobium pellet which is easy to make and economical to manufacture.
The method of accomplishing these and other objects will become apparent from the following description of the invention.
It is a feature of the present invention to provide a process by which nitride precipitation on tantalum and niobium pellets as a result of nitrogen doping is substantially eliminated. The process involves introducing nitrogen to tantalum and niobium pellets in an oxygen-free environment at a temperature range of from about 600-1400xc2x0 C. Generally, sintering temperature is higher than nitrogen doping temperature. The preferred temperature range is 700xc2x0 C. -1250xc2x0 C. Under these conditions, the absorption of nitrogen by the tantalum or niobium pellet is maximized. In order to prevent nitrogen phase precipitation, the cumulative nitrogen and oxygen content in the volume of nitrogen doped pellets is kept below the solubility limit of these gases in tantalum and niobium at room temperature. These impurities are uniformly distributed in the tantalum and niobium particles due to the annealing in vacuum of the nitrogen doped pellets at the same temperature as the doping temperature. This method of binding the nitrogen to the tantalum or niobium also prevents excessive oxygen levels in the pellet, thus increasing the reliability of the capacitor and reducing capacitor degradation.